Method of improving blast furnace performance using raw petroleum coke



United States Patent It is well known to those skilled in the art thatthe blast furnace, even in its highest development is by no means theeven-going, easily-regulated monster the casual ob- Server may take itto be. Although furnace operations are under better control now thanever before, such furnaces are still capable of acting in unpredictableways.

In general and as illustrated on page 62 of Brays Ferrous ProcessMetallurgy, published in 1954 by John Wiley and Sons, Inc., blastfurnaces may be considered as having five parts: (1) the bottom, (2) thehearth, (3) the bosh, (4) the stack, and (5) the furnace top.

The construction and operation of such furnaces are described in manystandard reference works on the subject such as the aforecited volume byBray or the Annual Proceedings published by the American Institute ofMining, Metallurgical and Petroleum Engineers on Blast Furnace, CokeOven, and Raw Materials, etc.

Raw materials including the burden are charged through the top of thefurnace. The burden or ore to be reduced may consist of raw screenedand'blended ores,

prepared sinter, or agglomerated ore concentrates such as pellets. Thisburden along with limestone and coke constitutes the charge. Suchadditional materials as open hearth slag, scrap, and roll scale may beadded in minor percentages as available.v

In a typical furnace operation the coke is consumed or gasified by meansof a preheated blast of air which is forced into the furnace underpressure through tuyeres at the top of the hearth. This gasification ofcarbon supplies the necessary reducing gases and heat to promote thechemical reactions of reduction and also melt the iron and slag formed.The preheated hot blast may range in temperatures from about 1000" toover 1800 F. subject to limitations of heating capacity and refractorylinings used. In addition the blastmay be modified or enriched withoxygen; steam; and gaseous, liquid, or solid fuels. As the coke ofsuccessive charges is consumed the furnace is replenished by addingcharges to the top and the finished iron (hot metal) and slag areperiodically drained or tapped from the hearth. Thus a charge willrequire about 8-12 hours to smoothly move to the hearth of a furnaceWhere the coke will be consumed and the hot metal and slag removed.

When a furnace fails to operate smoothly it is said to be hanging duringperiods when the burden fails to move downward in a normal and smoothmanner. This hanging is characterized by rapidly increasing blastpressures and lack of movement of the burden in the upper reaches of thestock column. Initially this hanging may be caused by severalconditions, either singly or in combination, such as:

(1) Fused slag may resolidify;

(2) Coke and burden fines may plug the void space and cause both archingof the solid material and resistance to flow of the gases;

(3) Redeposited carbon from the reaction may fill the voids betweenparticles of ore and thus impervious masses.

3,190,745 Patented June 22, 1%65 As the coke is gasified and the ironand slag is melted below the area in which the hanging occurs, a'largevoid is created especially when the furnace hangs for any appreciabletime. When the unsupported burden even- Iually breaks loose, the furnaceis said to slip. This slip is characterized by an explosive release ofhigh temperature gas along with the sudden movement of the burden. Thesudden release of the gas causes asharp increase in top gas temperatureand pressure usually causing the bleeders or safety valves to open. Theburden dropping into the bosh causes a marked cooling of the area sincethis burden has been improperly heated due to the poor gas-solid contactduring the period of blockage or hanging.

Recurrent slips of this nature lead to deposits along the periphery ofthe bosh. These scabs or scaffolds further interfere with'normal furnaceoperation by restrict ing the flow of the gases and also thecountercurrent flow of the burden. Thus scaffolds, once formed, causeirregular operation and promote further encrustation on the furnacewalls. 7

' As these scalfolds increase the furnace continually hangs and slipsand must be operated at greatly reduced blast volumes and productionrates. For example, one blast furnace with a 23 ft. diameter hearth and28,000 cubic feet working volume, but which had a substantial amount ofscaffolding, was producing only tons per day of hot metal as compared tonormal production of about 900 tons per day. This furnace had an in walltemperature fifteen feet above the bosh of 9001000 R, which was aboutSOD-600 F. below normal- Several different approaches to solving, orminimizing or eliminating such adverse phenomena, and for improvingfurnace performance have been suggestedor tried with varying degrees ofsuccess. For example, in some cases the quantity of blast has beenreduced in order to lower its buoyant effect, with the result that theweight of the material above the scaffolding incrustation is sometimesable to break the incrustation. This is referred to as a bosh slip.Sometimes the incrustationcan be removed'by the use of quartzSi0addition which acts on the deposits like rocksalt on ice. 'Sometimesextra metallurgical or foundry coke over the amount normally used hasbeen, added and found to be helpful. In extreme cases a number of holesmay be drilled at various levels of the furnace to blast or dynamite outthe incrustation or bridges, etc. An additional complicating factor inblast furnace operation is that the blast furnace operator must thinkabout 8 to 10 hours ahead of the furnace because any burden or chargechange will require that time to travel through the furnace and reachthe hearth where its effect will be mainly manifested.

It has now been found that a very elfective, efficient, and economicalway to restore a blast furnace to its normal stock movement, temperatureprofile, and hot metal output basis, after these have been reduced dueto such conditions as scaffolding or bridging or hanging, etc., is tocharge substantial quantities of raw petroleum coke to the slow movingcharge already contained in said furnace. Such raw petroleum cokedevelops very high and intense temperatures upon burning, resulting intemperatures sufliciently high to weaken the scaffolding by softening orincreasing the fluidity of the highly viscous slag. Also, because rawpetroleum coke has low ash content (typically about 0.3% as compared to7-14% ash, for normal furnace coke), upon burning, it contrib utes verylittle if any to slag build-up as does normal furnace coke upon burning.Further it permits a limestone burden adjustment which forms a morenearly finished slag in the bosh or primary melting zone.

The high temperature at which petroleum coke burns in relation tometallurgical coke is caused by its structure. Micro photographs showpetroleum coke to have very troleum coke will produce higherconcentrations of carbon dioxide and therefore the heat release issubstantially higher resulting in higher combustion temperature.

The process of this invention is illustrated by the following example.

EXAMPLE 1 A blast furnace having a hearth 23 feet in diameter and aworking volume of 28,000 cubic feet was sick" or cold for months, had anin-wall temperature fifteen feet above the bosh of 900-l000 F., and wasproducing only about 150 tons of hot metal per day. Thirty-two tons ofraw petroleum coke, having a volatile content of 12.5% and a particlesize of 100% 4 inch mesh and approximately 50% inch mesh, was chargedthrough the top of said furnace and added to the normal furnace chargealready therein. Within 48 hours the in-wall temperatures 15 feet abovethe bosh had increased by about 100-400 F. to 1000-1300 F., 1100 F.average. After this 48 hours, thirty-two more tons of the same rawpetroleum coke were charged into the furnace in the same manner. (Thefurnace was charged in a conventional manner during the intervening 48hours with ore, coke and limestone according to a typical blast furnacecharging cycle such as suggested on pages 73 and 74 of the volume byBray.) Within 24 hours the temperature of the furnace 15 feet above thebosh had risen to 1200 1400 F., 1300 F. average. Five days later 64 tonsof the same raw petroleum coke was added to the furnace and within 24hours the in-wall temperature 15 feet above the bosh averaged about 1500F. In other words, furnace temperatures were restored to normal; as wasalso hot metal output.

As is apparent from the foregoing example, the material employed toeffect this restoration to normalcy was utilized in the ore reductionprocess, the blast furnace was operated continuously while therestoration was taking placeand without the employment of extraordinarylabor or blasting, or extraneous additives.

The raw petroleum coke employed in this invention is typically of thetype produced from a delayed coker system and generally will have avolatile matter content between about 8 and about 15% with about 12 to14% being typical. It is preferable also that a substantial proportionof the raw petroleum coke employed be not too large nor too fine. Rawpetroleum coke particles which are retained on a inch screen and whichpass through a inch screen are typical of preferred particle sizes, butthis should not be interpreted as precluding the use of minorpercentages of raw petroleum coke coarser than 5 inches or finer thaninch. The ash content of the raw petroleum coke is preferably no higherthan about one percent.

The invention, although most applicable to blast furnaces, can beapplied to any shaft furnace, such as hot blast basic cupolas, in whichincrustation or build-up of viscous material along the in-walls is aproblem.

Having thus described the nature of my invention, but

7 being limited only by the appended claims with respect to its scope, Iclaim:

1. A method of assisting in restoring a shaft furnace to normaloperating temperature after the temperature of said furnace has beenreduced below said normal operating'temperature because of scaffolding,which comprises adding to the shaft furnace as extra coke, raw petroleumcoke, having a volatile matter content between about 8% and about 15%and an ash content less than about 1%, in an amount sufficient toincrease said reduced operating temperature toward said normal operatingtemperature.

2. A process according to claim 1 wherein over of the raw petroleum cokepossesses a particle size larger than one-eighth inch.

References Cited by the Examiner UNITED STATES PATENTS 2,184,318 12/39Ruzicka 42 3,058,821 10/62 Triska 75-41 OTHER REFERENCES Clements: BlastFurnace Practice, vol. III, published by E. Benn, Ltd., London 1929,pages 81-85.

Elliot et al.: Practical Ironmaking, published by United Steel Cos.Ltd., Shefiield, UK, 1959, pages 205206.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

1. A METHOD OF ASSISTING IN RESTORING A SHAFT FURNACE TO NORMALOPERATING TEMPERATURE AFTER THE TEMPERATURE OF SAID FURNACE HAS BEENREDUCEC BELOW SAID NORMAL OPERATING TEMPERATURES BECAUSE OF SCAFFOLDING,WHICH COMPRISES ADDING TO THE SHAFT FURNACE AS EXTRA COKE RAW PETROLEUMCOKE, HAVING A VOLATILE MATTER CONTENT BETWEEN ABOUT 8% AND ABOUT 15%AND AN ASH CONTENT LESS THAN ABOUT 1%, IN AN AMOUNT SUFFICIENT TOINCREASE SAID REDUCED OPERATING TEMPERATURE TOWARD SAID NORMAL OPERATINGTEMPERATURE.